The Comparison of Sorption and Solubility Behavior of Four Different Resin Luting Cements in Different Storage Media.

STATEMENT OF THE PROBLEM
Structural integrity and dimensional stability are the key factors that determine the clinical success and durability of luting cements in the oral cavity. Sorption and solubility of self-adhesive resin luting cements in food-simulating solutions has not been studied sufficiently.


PURPOSE
This study aimed to compare the sorption and solubility of 2 conventional and 2 self-adhesive resin-based luting cements immersed in four different storage media.


MATERIALS AND METHOD
A total of 32 disc-shaped specimens were prepared from each of four resin luting cements; seT (SDI), Panavia F (Kuraray), Clearfil SA Cement (Kuraray), and Choice 2 (Bisco). Eight specimens of each material were immersed in all tested solutions including n-heptane 97%, distilled water, apple juice, or Listerine mouth wash. Sorption and solubility were measured by weighing the specimens before and after immersion and desiccation. Data were analyzed by SPSS version 18, using two-way ANOVA and Tukey's HSD test with p≤ 0.05 set as the level of significance.


RESULTS
There was a statistically significant interaction between the materials and solutions. The effect of media on the sorption and solubility was material-dependent. While seT showed the highest values of the sorption in almost all solutions, Choice 2 showed the least values of sorption and solubility. Immersion in apple juice caused more sorption than other solutions (p≤ 0.05).


CONCLUSION
The sorption and solubility behavior of the studied cements were significantly affected by their composition and the storage media. The more hydrophobic materials with higher filler content like Choice 2 resin cement showed the least sorption and solubility. Due to their lower sorption and solubility, these types of resin-based luting cements are recommended to be used clinically.


Introduction
Various adhesive cements are being frequently used for cementation of indirect restorations. The use of resin cements facilitates the application of more conservative restorations such as porcelain inlays, veneers and resin-retained fixed dental prostheses. [1] Resin luting cements have the advantage of superior mechanical and physical properties when compared to the traditional luting cements. [2] Resin cements are classified in 3 groups of conventional (total-etch), self-etch, and self-adhesive resin cements. One of the disadvantages of using conventional resin luting cements is their multistep application which renders their quality for being technique-sensitive. [3] To overcome this problem, the new self-etch and self-adhesive luting cements are easier to use and require less clinical steps. This is owing to their composition which consists of monomers that have bonding as well as mineralizing capacities.
The clinical success and durability of luting cements in the oral cavity depend on different properties such as structural integrity and dimensional stability which are functions of water sorption and solubility. [4] When resin cements are exposed to moist conditions, substances such as unreacted monomers dissolve from the cements, where the lost mass is measured as solubility. [4] Sorption arising from the polymer matrix hydrolytically degrades the network structure, debonds the silanized filler and consequently influences the solubility of these materials. [4] The solubility behavior of resins is related to several factors such as unreacted monomers, size and chemical compositions of material, and chemical characteristics of the solvent. [3] In the oral cavity, restorations are often close to the gingival crevice and in contact with the oral fluids. Therefore, water sorption and solubility of these materials may have unwanted consequences during clinical use including degradation of the cement which can lead to fracture of the restoration, [2] marginal leakage, and the risk of secondary caries. [5] Water sorption of a polymer mainly occurs in the resin matrix, [6] and the absorbed water acts as a plasticizer and leads to degradation of fillermatrix interface, material discoloration, and aesthetic issues in the restoration. [7] Moreover, solubility produces toxic substances such as formaldehyde and methacrylic acid. Accumulation of these products along with the residual monomers, fillers, and residual activators due to the polymerization can be hazardous to the oral soft tissues. [8] Many studies have been conducted to evaluate the effect of different media on the physical and mechanical properties of resin-based materials. [4, 9-10] Yet, water sorption and solubility of self-adhesive resin luting cements in food-simulating solutions has not been widely studied. The aim of this study, therefore, was to compare the sorption and solubility of four adhesive resin cements in food-simulating solutions and Listerine mouth wash. The null hypothesis is that there is no difference between the conventional and self-adhesive resin cements in relation to sorption and solubility. Table 1 shows the description of all resin luting cements used in this study. For each type of cement, 32 discsshaped specimens were prepared using a polyethylene For the completion of an additional acid-base setting reaction, the specimens were stored at room temperature for 24 h. Prior to removing the specimens from the mold, the excess material was removed through gentle wet manual grinding on both sides by using 1200-

Statistical Analysis
The data were statistically analyzed by SPSS software

Results
The means and standard deviations for sorption and solubility are shown in Table 2

Discussion
Clinically, resin luting cements are indicated when greater retention is needed [11] and are used for cementation of ceramic restorations as they increase the durability of the cemented ceramics. [2] Since the cements are inevitably exposed to oral fluids, they should withstand deterioration after exposure to changes in the oral environment. Cement degradation has been attributed to marginal leakage, hypersensitivity, secondary caries, and loss of restoration retention. [12] Degradation of materials in the oral cavity is composed of two components; mechanical and chemical. [10] It is reasonable to assume that the chemical component initiates the resin cement degradation. [13] In this study, four different immersion solutions were selected to simulate the oral environment alterations to which the restorative substrate is exposed.
According to the results of the present study, there  (Table 2). Resin-based materials with lower filler content also displayed higher sorption. [14] Furthermore, cements with greater matrix portion are more prone to hydrolysis and subsequent degradation.
[15] On the contrary, the sorption and solubility of Choice 2 were less than all other cements, which could be attributed to its higher filler loading (78.9 Wt%) than the other tested resin cements. [16] In addition, the selfadhesive cements are produced with hydrophilic acidic monomers such as carboxylic acid or phosphoric acid groups, which will make the cement more susceptible to sorption and subsequent solubility. In terms of marginal adaptation, ceramic restorations exhibited more marginal opening than the restorations with metal fitting surfaces. [18] The implications of marginal opening are greater exposure for cementation material to oral environment which could directly increase the amount of deterioration. Therefore, it is reasonable to employ a resin luting cement with low sorption and solubility for cementing all ceramic restorations. A direct relation was reported between water sorption and solubility of dental resin; the solubility increased as water sorption increased. [19] This study illustrated the significant effect of surrounding media on the cement solubility and sorption.
Acidic environments such as citric acid and ascorbic acid of apple juice appeared to be harsh environments and clearly contributed to the greater solubility of resin cement. This confirms the findings of several studies on the effect of low pH. [3,5] For example, Marghalani found that immersion of resin cements in lactic acid increased the sorption and solubility. [3] The negative effect of acid is attributed to the vulnerability of resin cement matrix to hydrolysis after being exposed to hydrogen ions. [15] The presence of hydrogen ion accelerates the catalysis for the ester groups of dimethacrylate monomers. [20] It causes degradation of the polymer crosslinking and softens the resin cement. [20] Eventually, monomer release is enhanced and with prolonged acidic exposure, the external filler particles are released from the cement mass. Although the solubility of resin cement appears to increase in acidic medium, Yoshida et al. and Hamouda reported that resin cements still exhibit significantly less solubility than the conventional acid-base cements. [5,21] Therefore, the clinical significance of acidic deterioration is yet to be determined since constant exposure of exposed resin cement to acid in oral environment is very unlikely due to the continuous protective buffering capacity. Disc-shaped specimens in constant medium are more susceptible to degradation due to constant effect of the solution on a large surface and when compared to minimally-exposed cement at the margin of restoration, the magnitude of sorption and solubility would likely be less.
As proposed by earlier researches, this study used heptane as fatty food simulator. [29][30] Generally, heptane solution was found to increase the sorption and solubility of most cements. This confirms the previous researchers' findings. [29][30] The effect of heptane solution was attributed to the ease of penetration into resin matrix [29] and the subsequent plasticizing effect.
[30] The susceptibility of resin matrix to softening can explain why Choice 2 was the least affected by heptane solution.
It must be emphasized that there was a prominent variability in the solubility values ranging from negative to positive values. The significant variation could be a result of the experimental set-up of this study. All materials were subjected to some, but variable, degrees of dissolution after immersion. The reason of different mass changes after the second desiccation could be related to the equilibrium of water uptake and actual mass loss which was different for each material. Some of the absorbed water was firmly bounded to the resin matrix and could not be completely removed. [31] Therefore, after the desiccation process which followed water storage, only that amount of water which was loosely connected to the matrix was removed. As each material behaves differently in each condition, the overall readings differ.
The outcome of this study should be carefully in-

Conclusion
Within the limitations of this study, a significant material-dependent interaction was detected between the solution and material (p< 0.05).
Among the solutions, heptane and distilled water affected the materials significantly. Among the materials, seT showed the highest sorption in all solution; while, Choice 2 showed the lowest.
For solubility, the significant interaction between the materials and solutions was even stronger (p< 0.002) than that of sorption (p< 0.05). In distilled water, the differences in solubility means were significantly higher for Choice 2 and Panavia F compared with seT and Clearfil SA.